The present invention relates generally to the field of solid state integrated filter circuits and more particularly to a low-corner frequency high pass filter circuit in an integrated circuit.
In order to build a high pass filter with a pole at very low frequencies it is necessary to have large a resistance and capacitance in the circuit. In integrated circuits it is extremely difficult to build large capacitors and resistors. In addition, it is expensive to build large value resistors and capacitors which require a lot of die area, as cost is proportional to die area.
The cost issue and other challenges might be better understood with an example. A typical high-pass filter might require a corner frequency of 100 Hz, which in a discrete implementation might use a 160 kiloOhm resistor and a 10 nanoFarad capacitor. If this was implemented in an integrated circuit, the parameters that might be typical are 1k ohm per 10 micrometers2 of area for an xe2x80x9cn-wellxe2x80x9d resistor, and 0.1 femtoFarad per micrometer2 for a metal-to-metal capacitor. Thus, this RC filter would require approximately 100 mm2 of die area, which is extremely large for an IC, and very expensive. In an attempt to reduce the die area, one might make the resistor larger and the capacitor smaller. However, the large resistor causes offset problems. If the resistor is raised to 16 megOhms, and the capacitor reduced to 100 picoFarads, then the die area required is approximately 2.6 mm2, which is still very large. Even worse though, is the fact that circuit offsets become very difficult. The 16 megOhm resistor must not be loaded by a small impedance of the circuit following this low-pass. If it is loaded even by a 1600 megOhm resistor, then there will be a 1% error; but such a high load impedance is extremely challenging, and nearly impossible. If there is a tiny leakage current of only 10 nanoAmps through the 16 megOhm resistor, then the result is 160 milliVolts of offset, which is much larger than many important analog signals in ICs.
This example shows the fundamental problems of creating a long time constant high-pass filter on an integrated circuit. Usually at least 2 problems occur: 1) a large use of die area is required, or 2) large offset voltage errors are introduced.
Thus there exists a need for a low-corner frequency high pass filter circuit that does not require large die areas, and that is capable of not introducing significant voltage offset errors.